--- src/Makevars.in
+++ src/Makevars.in	2024-04-30 22:28:03.000000000 +0800
@@ -1,5 +1,5 @@
 PKG_CPPFLAGS=-I. -Icommon -Ikms -Iutf8proc \
-	-D_XOPEN_SOURCE=700 -D_DEFAULT_SOURCE \
+	-D_XOPEN_SOURCE=700 -D_DARWIN_C_SOURCE \
 	-DKMS_MESSAGE_ENABLE_CRYPTO -DKMS_MESSAGE_ENABLE_CRYPTO_LIBCRYPTO \
 	-DBSON_COMPILATION -DMONGOC_COMPILATION -DOPENSSL_SUPPRESS_DEPRECATED @cflags@